It is known in the art relating to internal combustion engines that by operating an engine with a less than stoichiometric (lean) mixture of fuel and air, efficiency of the engine is improved. This means that for a given amount of work performed by the engine, less fuel will be consumed, resulting in improved fuel efficiency. It is also well known that reduction of NOx emissions when the fuel rate is lean has been difficult to achieve, resulting in an almost universal use of stoichiometric operation for exhaust control of automotive engines. By operating an engine with a stoichiometric mixture of fuel and air, fuel efficiency is good and NOx emission levels are reduced by over 90% once the vehicle catalyst reaches operating temperatures.
Recent developments in catalysts and engine control technologies have allowed lean operation of the engine, resulting in improved fuel efficiency and acceptable levels of NOx emissions. One such development is a NOx adsorber (also termed a “lean NOx trap” or “LNT”), which stores NOx emissions during fuel lean operations and allows release of the stored NOx during fuel rich conditions with conventional three-way catalysis to nitrogen and water. The adsorber has limited storage capacity and must be regenerated with a fuel rich reducing “pulse” as it nears capacity. It is desirable to control the efficiency of the regeneration event of the adsorber to provide optimum emission control and minimum fuel consumption. It is further desirable to control the efficiency of the regeneration event of the adsorber to provide optimum emission control and minimum fuel consumption while at the same time minimizing or eliminating altogether any adverse impact on driveability. Various strategies have been proposed.
Commonly assigned U.S. Pat. No. 6,293,092 to Ament et al. entitled “NOx adsorber system regeneration fuel control” discloses a method for controlling regeneration fuel supplied to an internal combustion engine operating with a lean fuel-air mixture during sequential rich mixture regeneration events of a NOx adsorber in which NOx emissions collected by the adsorber are purged to provide optimum emissions control and minimum fuel consumption. The method monitors the exhaust gases flowing out of the adsorber during the regeneration event to detect when the fuel-air mixture to the engine is within an excessively lean or rich range. When the sensed exhaust gases contain an excessively lean fuel-air mixture, fuel is increased to the engine. Fuel is decreased when the sensed exhaust gases contain an excessively rich fuel-air mixture. The fuel can be increased or decreased by adjusting the duration or fuel rate of the regeneration event. U.S. Pat. No. 6,293,092 is hereby incorporated by reference.
In the art related to spark-ignition direct-injection (SIDI) engines, it is known to operate the engine in a stratified charge mode (very lean operation) in a lower range of engine output and in a homogeneous mode (less lean, stoichiometric, or rich of stoichiometric operation) in a higher range of engine power output with an intermediate zone wherein the cylinders operate in a combination of stratified charge and homogeneous charge combustion. In the stratified charge mode, the fuel is injected during the piston compression stroke, preferably into a piston bowl from which it is directed to a spark plug for ignition near the end of the compression stroke. The combustion chambers contain stratified layers of different air/fuel mixtures. The stratified mode generally includes strata containing a stoichiometric or rich air/fuel mixture nearer the spark plug with lower strata containing progressively leaner air/fuel mixtures. In the homogeneous charge mode, fuel is injected directly into each cylinder during its intake stroke and is allowed to mix with the air charge entering the cylinder to form a homogeneous charge, which is conventionally ignited near the end of the compression stroke. The homogenous mode generally includes an air/fuel mixture that is stoichiometric, lean of stoichiometric or rich of stoichiometric.
Typically, there is a first range of air-fuel ratios within which stable combustion can be achieved in the stratified charge mode, such as between 25:1 and 40:1, and a second range in which stable combustion can be achieved in the homogeneous mode, such as between 12:1 and 20:1. Therefore, there is typically a significant gap between the leanest air-fuel ratio of the homogenous mode (in this example 20) and the richest air-fuel ratio of the stratified mode (in this example 25). This gap poses a number of challenges in selecting an appropriate operating mode and controlling the engine during transitions between operating modes. For example, careful control of engine operation is necessary to deliver the demanded torque without adversely affecting driveability when switching from the stratified to the homogenous mode or vice versa.
It is known in the art to coordinate valve timing during mode transitions to reduce engine torque variations. Methods to control individual engine variables during normal, single-mode operation as a lean NOx trap regeneration engine control strategy have also been proposed. But control of individual engine parameters can result in unacceptably rough operation. Transient control of fuel injection timing similar to other variables has also been proposed. But this can produce oscillatory behavior resulting from engine misfire.
Commonly assigned co-pending U.S. patent application Ser. No. 10/812,584 filed Mar. 30, 2004, the disclosure of which is hereby incorporated by reference herein in its entirety, describes a method to control a direct-injection gasoline engine during LNT regeneration events thereby improving driveability by timing transitions to homogeneous operation in accordance with fuel/air equivalence ratio considerations. Further, commonly assigned co-pending U.S. patent application Ser. No. 10/812,467 filed Mar. 30, 2004 also directed to a control strategy for lean NOx trap regeneration whereby the number of regeneration events carried out when a lean burn SIDI engine is otherwise running in a stratified mode are minimized, is hereby incorporated by reference herein in its entirety. However, lean NOx trap regenerations are still required under some stratified mode operating conditions and there is usually potential for undesirable degraded driveability during the occurrence of such regeneration events.
Therefore, there remains a need in the art for further advances in the control of engine operation during lean NOx trap regeneration. There further remains a need in the art for methods providing comprehensive coordinated control of engine operation during mode transitions associated with LNT regeneration that enable LNT regeneration to occur without adversely impacting driveability or NOx emissions at the tailpipe, particularly for mixed mode spark-ignition direct-injection (SIDI) engines.